Simple Fluorescence Labeling Method Enables Detection of Intracellular Distribution and Expression Level of Retinoid X Receptors

There is no straightforward method to visualize the intracellular distribution of nuclear receptors, such as retinoid X receptors (RXRs), which are trafficked between the cytosol and nucleus. Here, in order to develop a simple fluorescence labeling method for RXRs, we designed and synthesized compound 4, consisting of an RXR-selective antagonist, CBTF-EE (2), linked via an ether bond to the fluorophore nitrobenzoxadiazole (NBD). Compound 4 is nonfluorescent, but the ether bond (-O-NBD) reacts with biothiols such as cysteine and homocysteine to generate a thioether (-S-NBD), followed by intramolecular Smiles rearrangement with an amino group such as that of lysine to form a fluorescent secondary amine (-NH-NBD) adjacent to the binding site. Fluorescence microscopy of intact or RXR-overexpressing MCF-7 cells after incubation with 4 enabled us to visualize RXR expression as well as nuclear transfer of RXR induced by the agonist bexarotene (1).